Removed conversion from youtube closed caption format to srt since youtube api supports the 'srt' format
Added new option '--only-srt' to download only the subtitles of a video
Improved option '--srt-lang'
 - it shows the argument in case of missing subtitles
 - added language suffix for non-english languages (e.g. video.it.srt)
Added new option '--all-srt' to download all the subtitles of a video.
Only works in youtube for the moment.
Refactor subtitle options from srt to the more generic 'sub'.
In order to be more consistent with different subtitle formats.
From:
* --write-srt to --write-sub
* --only-srt to --only-sub
* --all-srt to --all-subs
* --srt-lang to --sub-lang'

Refactored also all the mentions of srt for sub in all the source code.
Added new option '--sub-format' to choose the format of the subtitles to downloade (defaut=srt)
Added new option '--list-subs' to show the available subtitle languages
Fixed typo in error message when no subtitles were available.
add '--write-srt' and '--srt-lang' aliases for backwards compatibility
Merge pull request #699 by @iemejia

Removed innecesary function to convert subtitles, improved use of the youtube api